Research Accomplishment Reports 2007

Calcium and Phosphorus Nutrition of Pregnant and Lactating Mares

L.M. Lawrence
Department of Animal and Food Sciences

 

Project Description

Horses are the leading agricultural crop in Kentucky. The nutritional management of broodmares impacts their productivity and longevity. Poor nutrition can reduce reproductive efficiency and decrease longevity. This study focused on factors that affect the mineral requirements and pasture-based nutritional programs of pregnant and lactating mares.

Markers of bone turnover were measured in mares during late gestation and early lactation during three foaling seasons. All mares were fed calcium adequate diets. Blood samples were obtained during the last month of gestation, on the day after foaling and at various times during lactation. Osteocalcin, an indicator of bone deposition, was measured using Elisa. Carboxyterminal telopeptide of type I collagen (ICTP), was measured as an indicator of bone resorption using radioimmunoassay. In the third foaling season, samples were also collected from non-pregnant mares. Data were examined by analysis of variance for effects due to sample time. Osteocalcin concentrations did not decrease at foaling. In year 1, osteocalcin concentrations were elevated (P<0.05) at 1 month of lactation, but a similar response was not observed in year 2 or 3. ICTP was higher (P<0.05) on the day after foaling and at 2 weeks after foaling than during the last month of gestation. ICTP concentrations returned to prefoaling levels by 3 months of lactation. The osteocalcin: ICTP ratio was calculated as an indicator of the balance between bone mineralization and bone resorption. In all years, the osteocalcin: ICTP ratio was reduced in mares on the day after foaling (P<0.05). When control mares were samples simultaneously with pregnant/lactating mares, neither/or osteocalcin ICTP changed over time.

These data are consistent with results in other species and indicate that mares mobilize bone mineral at the onset of lactation even when they are fed diets containing recommended levels of calcium and phosphorus during late gestation and early lactation. The effect of calcium and phosphorus supplementation above recommended levels was evaluated in mares in late lactation and post-weaning. Calcium and phosphorus supplementation did not alter the indicators of bone turnover in mares during late lactation and did not alter milk calcium or phosphorus (P> 0.05).

These results suggested that the current calcium and phosphorus recommendations for mares in late lactation are probably adequate. There was also no difference in markers of bone mineral turnover in mares after weaning, However, the available forage provided more calcium to the control mares than expected. A follow-up study was performed to characterize seasonal variations in the nutrient composition of pasture. Five years of monthly pasture samples were analyzed and reveal variations in calcium and phosphorus content as well as the calcium: phosphorus ratio. Calcium concentrations were lowest when plants were actively growing in the spring months. These data suggested that the spring may be a critical time for mineral supplementation in pastured broodmares.

Impact

This research has identified key factors influencing the calcium and phosphorus nutrition of pregnant and lactating mares. These are the first studies to document that mares in late gestation and early lactation will mobilize skeletal mineral to support the developing fetus and the demands of milk production. Mobilization of bone mineral appears to occur even when calcium adequate diets are fed. Providing calcium and phosphorus supplementation beyond the current recommended levels in late lactation did not appear to enhance re-mineralization of the maternal skeleton. Calcium and phosphorus supplementation of mares grazing fall pastures in the immediate post-weaning period did not appear to alter the balance between bone mineralization and bone mobilization. However, the fall pasture provided more than the currently recommended amounts of calcium and phosphorus for non-lactating mares.

Analysis of five years of monthly pasture samples indicated that seasonal variations in the concentration of calcium and phosphorus are marked. Calcium concentrations are highest in the fall which coincides with the post-weaning period. Therefore, calcium concentrations in fall pasture in Kentucky appear to provide adequate calcium intakes for mares immediately after weaning. However, calcium concentrations are lowest in the spring, when milk production is highest. Therefore, calcium supplementation of Kentucky broodmares in pasture-based nutritional programs is essential. Failure to meet calcium and phosphorus requirements will result in increased mobilization of bone mineral during late gestation and lactation and may impair remineralization of the skeletal mass after weaning.

Mares used for commercial production may be subjected to many consecutive gestation-lactation cycles. Failure to replenish bone mineral would be expected to reduce skeletal integrity and reduce longevity. This findings of this project enhance our understanding of bone mineral turnover in broodmares and improved our ability to formulate appropriate nutritional programs for all stages of gestation and lactation.

Publications

Cassill, B.D., Hayes, S., Ringler, J., Janicki, K. and Lawrence, L. 2004. The effect of dietary calcium on indicators of bone turnover in mares. Journal of Animal Science 83(Supplement 1):330

Cassill, B., Hayes, S., Ringler, J. and Lawrence, L. 2005. Changes in serum markers of bone turnover during pregnancy and lactation. Proc. Equine Science Soc. 19:63-64

Cassill, B., Lawrence, L., and Collins, M. 2005. Acceptability of red clover hays to horses. Proc. Equine Science Soc. 19: 360-361.

Powell, D. and Lawrence, L. 2005. A comparison of two different weaning methods on foal body weight gain. Proc. Equine Science Soc. 19:333-334.

Pratt, S.E., Lawrence, L.M., Warren, L.K. and Powell, D. 2005. The effect of acetate infusion during exercise in the horse. J. Equine. Vet. Sci 25:260-271.

Ringler, J., Cassill,B.,Hayes, S. and Lawrence, L. 2005. Comparison of in vitro digestibility estimates using the Daisy II incubator to in vivo digestibility estimates. Proc. Equine Science Soc. 19:43-44.

Ringler, J.,Cassill,B., Hayes,S. and Lawrence, L. 2005. Effect of incubation time on in vitro estimates of DM, NDF and ADF digestibility obtained using equine feces as inoculum. Proc. Equine Science Soc. 19:307-308.
Ringler, J.,Cassill, B., Hayes, S., Stine,J. and L. Lawrence. 2005. Grazing preferences of horses for different cool season grasses. Proc. Equine Science Soc. 19:319-320.

Cassill, B., Hayes, S., Ringler, J. and Lawrence., L. 2006. The effect of sedation on serum cortisol concentrations in mares during weaning. J. Anim. Sci. 84(Supplement 1):329

Lawrence, L., Hayes, S., Allman, R. and Rich, G. 2006. Seasonal variation in cool season grasses. J. Anim. Sci. 84(supplement 1):331.

Watson, K., Smith, Jr. S.,and Lawrence. 2007. Plant factors affecting preference of horses for cool season grasses. Proc. Equine Science Soc. 20:17-18.

Cassill, B., Hayes, S., Ringler, J., Stine, C.J., Watson, K., and Lawrence, L. 2007. Glycemic responses of mature mares to a reduced starch concentrate. Proc. Equine Science Soc. 20:238.

Ringler, J., and Lawrence,L. 2008. Comparison of Thoroughbred growth data to body weights predicted by the NRC. J. Equine Vet Science (in press)